Circuit boards such as printed circuit boards (PCBs) typically include plated through holes as interconnect structures to electrically couple components. A conventional through hole may traverse an entire cross-section of a circuit board, wherein electrical traces extending from a via portion of the through hole may act as an electrical path to electrically couple components on the PCB. In some cases, the intended signaling portion of a via may cover less than the entire length of the through hole, wherein the remaining unintended signaling portion of the through hole may be considered a stub portion. A related art plated through hole is shown in FIG. 1. A via portion 4 is shown between electrical traces 6 and 8 while a stub portion 9 extends below electrical trace 8. Electrical traces may interconnect a transition between a surface layer and an internal layer or between two internal layers; the closer the internal routing layer is to the surface layer, the longer the stub portion will be.
The impedance of the stub portion may be approximated by that of a transmission line with an open termination:ZOC=−jZ0cot(β1)
where j is an imaginary number, Z0 is a characteristic impedance of the system, β is a propagation constant, and 1 is the stub length.
For a frequency in the GHz range and a stub length in the millimeter range, the impedance will maintain a negative imaginary value, which is equivalent to a capacitance in shunt with a primary electrical interconnect. This condition may be represented as an equivalent circuit that includes a channel and a capacitance representing a stub portion. The stub portion significantly changes the impedance profile of the through hole, causing severe reflections, resonance, and additional loss. Consequently, the structure is not useful for high speed links. To reduce these effects, the stub may be removed through drilling. However, stub removal may add cost and decrease the mechanical integrity of the circuit board. Micro-via and/or buried via technologies may be used to remove the stub as well. However, these technologies may add significant cost to manufacturing the circuit board.